Control means for grinding machines

ABSTRACT

Grinding mills for wood wherein the wood is fed to a grinding stone require regulation to produce pulp of good quality. A signal measuring the friction coefficient between the wood and grinding stone is derived and this signal controls the wood feeding to keep the friction coefficient constant.

United States Patent 1 Sandblom et al.

[ Dec. 4, 1973 CONTROL MEANS FOR GRINDING MACHINES [75] Inventors: Henry Sandblom; Vilmos Torok,

- both of Vasteras, Sweden [73] Assignee: Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden 22 Filed: Sept. 9, 1971 21 Appl. No.: 179,011

[30] Foreign Application Priority Data Sept. 10, 1970 Sweden 12296/70 [52] US. Cl. 241/34 [51] Int. Cl. B02c 25/00 [58] Field of Search 241/34, 35

[56] References Cited UNITED STATES PATENTS 1,871,499 8/1932 Crago 241/35 X 1,691,101 11/1928 Winkler 241/35 1,721,863 7/1929 Grossler 241/34 X 1,851,134 3/1932 Schnitzer.... 241/34 X 1,873,982 8/1932 Rusterholz.. 241/34 X 2,001,543 5/1935 Payne 241/34 2,156,736 5/1939 Perry 241/34 X Primary ExaminerGranville Y. Custer, Jr.

[57] ABSTRACT Grinding mills for wood wherein the wood is fed to a grinding stone require regulation to produce pulp of good quality. A signal measuring the friction coefficient between the wood and grinding stone is derived and this signal controls the wood feeding to keep the friction coefficient constant.

3 Claims, 2 Drawing Figures CONTROL MEANS FOR GRINDING MACHINES BACKGROUND OF THE INVENTION The present invention relates to grinding machines,

such as grinding machines for wood, having at least one 5 motor driven member for feeding the material to be ground to the grinding member.

Such machines are already known, inter alia through the Swedish Pat. No. 326,627, to which British Pat. No. 1,241,330 corresponds, which shows a grinding machine for wood with a grinding member and a feeding member, each with its own driving motor, in which current, power, speed, torque or some other value dependent on the load of the respective motor or driving shaft, is supplied to a transducer for the emission of an analog signal or a pulse train corresponding to the corresponding value measured. Each of these signals or a combination thereof is supplied to a comparison means for comparison with a desired value signal to produce a difference signal. The two signals and/or the combined signal are supplied to a counter in order to obtain a measurement of the energy, quantity fed, etc., and if a certain value is exceeded the difference signal is arranged to block the input signals to the counter or some of the signals, in order to correct for irregular feeding. Thus a value is immediately obtained for the feeding, etc., without having to correct for various error factors.

This is therefore an extremely practical way of measuring the feeding rate or specific consumption of energy, for instance, and the latter is not the least important information for-the quality of the grinding pulp. The quality of the grinding pulp is extremely difficult to regulate and for such regulation it is necessary to obtain a value very quickly which is characteristic of the quality and which can be used to regulate the machining in order to achieve uniform quality. Known means for such measurements have so far been much too slow for regulating purposes.

SUMMARY OF THE INVENTION The invention aims at a solution of these and other similar problems in grinding mills for wood pulp and the like and is characterized in that a signal obtained by measuring the friction coefficient between the wood and grinding member is supplied to a control device for the motor or motors of the feeding member in order to keep the friction coefficient constant. In this way there is a good chance of rapid and effective regulation of the desired quality of the pulp.

Mathematically the invention may be expressed as follows: If the normal pressure perpendicular to the centre of the grinding member is called N and the frictional force of the wood against the grinding member (grinding stone) is called F, the friction coefficient p. is obtained as follows:

N"p.= F

[L FIN The friction coefficient is not constant but varies with The expression specific grinding pressure (p) may also be introduced, in which case if the contact surface of the grinding material is s.

N/s P or P s -p. (P) F (u also varies with p). If p s N and F are now measured and the quotient F/p's is obtained, this gives a measurement of the variable friction coefficient. The specific friction force (1) F/sfs/p's =f/p is thus obtained. If p.(p) is kept constant then p will also remain constant and a uniform pulp quality is obtained.

THE DRAWINGS The invention is further exemplified in the accompanying FIG. 1 which shows schematically the principles of the invention applied to a grinder for grinding wood.

The FIG. 2 shows schematically a pocket grinder having two plun'gers.

PREFERRED EMBODIMENT FIG. 1 shows a grinding wheel or stone 11 driven by a synchronous motor 12 or some other type of motor, for example a water turbine, with measurable torque. The torque can be measured by measuring current or shaft torque, for example by means of a so-called torque transducer of the TORDUKTOR type (Swedish Pat. No. 166,632 to which U.S. Pat. No. 3,01 1,340 corresponds). The power transducer 16 therefore forms the torque gauge and its output signal gives a value F, i.e., for the friction force at the grinding stone. This power transducer is illustrated as comprising the amplifier 16 fed by a voltage derived from the voltage divider 16a comprising a resistance through which the electric power to the synchronous motor 12 is fed and to which the connections 16b are made to pick off a convenient electric voltage for the amplifier input. Therefore, the amplified electric voltage output F of the amplifier is an analog of the motors torque. Wood or other material for grinding is fed down by two transport devices 21 in the form of motor driven conveyor belts. These are driven by a motor 13 via a gear transmission 14 and this motor 13 may be electric or hydraulic and have variable speed. It may, for example, be fed via a controlled converter 15 (thyristor) fed from an AC network and controlled as follows.

The load on the grinding stone 11 can be measured in many ways: for example by means of a load cell 17 of the PRESSDUKTOR type (Swedish Pat. No. 151,267 to which U.S. Pat. No. 2,895,332 corresponds), the output signal (N) of which will thus be proportional to the normal force i.e., the force normal to the axis of the grind wheel, and which is supplied together with the friction force signal (F) from 16 to an allocating device 18, the output signal of which will be proportional to PIN or force divided by pressure, and thus to the friction coefficient. The load cell 17 as usual has an output which is an electric voltage N analog of the pressure loading on the shaft of the stone wheel 11 and therefore of the pressure the wood exerts on this wheel which is, in turn, due to the speed of the wood feedingmotor 13 which is controlled through the thyristor 15. In a summation or differentiating device 19 the friction coefficient signal is compared with a desired value from a desired value signal source 20 and a difference signal is obtained (or quotient if a quota device is used at 18), which is supplied to a proportionally or integrally operating control amplifier 22 of a type known per se. The output signal from this is supplied to the control circuit of the thyristor 15 and thus controls the current supply to the motor 13 and consequently its speed and the speed at which the wood is fed. If, therefore, due to altered friction between the wood and grinding stone 11, the torque in the motor 12 alters, the quotient F/N is altered and the feeding rate is altered so that the friction force is restored.

The device 18 may be any of the commercially available devices. As is well known, such a device, by analogy, may be represented by a potentiometer with the electric voltage signal F fed through its resistance and the electric voltage signal N fed to a motor moving its arm contactor in proportion to the signal N, which is also an electric voltage, the quotient being taken from the arm. However, modern forms of such devices used in analog computerized circuitry, are electronic devices.

The device 19 may be any of the commercially available operational amplifiers that produce an output signal proportional to the sum of the input signals. As in all analog circuitry, the signals are electric voltages. Thus, one input signal is here the quotient of the torque and pressure analog voltages representing the friction coefficient value, and the other input signal is obtained from the reference signal source which is, as is usual, shown as a potentiometer 20 from which the reference electric voltage is picked off. The value of this reference voltage is that known to provide the desired quality of ground pulp, or in other words, the friction coefficient between the wood and wheel 11 which provides the best quality.

Conventionally, the amplifier 22 can be the usual operational amplifier having capacity controlled feedback so that its output is proportional to the integral of its input signal. The thyristor to which its output signal is fed may be of the usual kind employed to control the speed of an alternating current motor.

FIG. 2 shows a plunger grinding mill having two plungers 23, 24 for feeding the wood. The driving member of each of these feeding devices can be controlled in the same way as in the device according to FIG. 1. The following can be seen from the force diagram:

h is the horizontal component of normal force k v is the vertical component of normal force k b and v are the same for k k corresponds to plunger 23 and k to plunger 24 H h h,

k V-cosai H-sina lsinoz -cosnz -+sinz -cosoz k V-cosoz H-sina /sina,-cosa +sinoz -cosoz V and H can be measured and from this k and k can easily be calculated, this values corresponding to the friction force at plungers 23 and 24, respectively.

Devices according to the above can be varied in many ways within the scope of the following claims. For instance, the speed of the machining member may also be controlled, separately or together with the feeding member. It should be noted that in connection with the above, the feeding channels 23 and 24 are directed downwardly towards the center of the grinding wheel.

The device also makes it possible to measure the quantity of wood fed. Since the specific grinding pressure is kept constant the total grinding pressure (normal pressure) will be proportional to the contact surface s. The corrected (correct) quantity fed will then be in cubic meters of wood, solid measure: m const.- N v m corrected quantity fed N p s v feeding rate const. l/p

It should be possible to obtain p by calibration methods so that the quantity of wood fed is measured in cubic meters solid measure over a period of time.

If on the other hand an arbitrary constant is used, then the following value is obtained which is also very useful:

m' const/p N- v const' s v The determination of p for various types of wood or methods of grinding will probably provide information of considerable interest concerning the grinding process in a mill.

The correct specific energy consumption can also be obtained: kWh/m (corr) see Patent specification No. 326,627. The filling ratio is obtained as constant 1: m /v (constant)" N a value which could be used for supervising the grinding mill or the operation/alarm systems of the grinding mill.

We claim:

1. A grinding mill including a grind wheel rotatively driven by a motor and means for feeding objects to be ground to said wheel and driven by a controllable speed means; wherein the improvement comprises means for measuring the value of the friction coefficient between said wheel and the objects fed thereto while said wheel is driven by its motor, and means for controlling the speed of said controllable speed means automatically in response to said value to keep said friction coefficient value substantially constant, and in which said measuring means includes means for measuring the value of the torque of said wheel s motor and means for measuring the value of the pressure of said objects on said wheel, said control means for said controllable speed means being automatically responsive to the value of the quotient of the first-named value divided by the second-named value.

2. The mill of claim 1 having selectively controllable means for modifying said quotient value to thereby set the value of said substantial constant friction coefiicicut.

3. The mill of claim 3 having a plurality of said feeding means each with controllable speed driving means and forming feeding channels directed towards the center of said grinding wheel, said means for measuring the value of the pressure of the object on the wheel being responsive only to the horizontal component of said pressure as to each of said channels, and said control means for the feeding means driving means in each instance being automatically responsive to the value of the quotient of said measure of said torque value divided by said horizontal component pressure value.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,77 7 Dated December 4, 1973 Henry sandblom and Vilmos Torok Patent No.

Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 54, after "claim", cancel "3" and replace with --l--.

Signed and sealed this 7th day of May 197E.

(SEAL) Attest: EDWARD PLFLETCifliIRJR. 0. MARSHALL DAMN Attesting Officer 7 Commissioner of Patents USCOMM-DC 60876-P60 r: us. GOVERNMENT PRINTING OFFICE: I90 0-386-3,

FORM PO-IOSO (10-69) 

1. A grinding mill including a grind wheel rotatively driven by a motor and means for feeding objects to be ground to said wheel and driven by a controllable speed means; wherein the improvement comprises means for measuring the value of the friction coefficient between said wheel and the objects fed thereto while said wheel is driven by its motor, and means for controlling the speed of said controllable speed means automatically in response to said value to keep said friction coefficient value substantially constant, and in which said measuring means includes means for measuring the value of the torque of said wheel''s motor and means for measuring the value of the pressure of said objects on said wheel, said control means for said controllable speed means being automatically responsive to the value of the quotient of the first-named value divided by the second-named value.
 2. The mill of claim 1 having selectively controllable means for modifying said quotient value to thereby set the value of said substantial constant friction coefficient.
 3. The mill of claim 3 having a plurality of said feeding means each with controllable speed driving means and forming feeding channels directed towards the center of said grinding wheel, said means for measuring the value of the pressure of the object on the wheel being responsive only to the horizontal component of said pressure as to each of said channels, and said control means for the feeding means'' driving means in each instance being automatically responsive to the value of the quotient of said measure of said torque value divided by said horizontal component pressure value. 